The present invention relates generally to solder reflow assembly techniques of elements or devices to substrates.
The attachment of elements to a substrate, for example a printed circuit board, is generally achieved by the use of solder.
One technique is to insert the leads of the device through the printed circuit board and process the printed circuit board over a wave of solder or dip the ends in a solder bath. The solder adheres to the leads and the boards, and when cooled, secures the leads to the board and electrically connects the leads to the printed circuit portions of the board. Another technique of soldering elements to substrates has included immersion of the substrate and the element in a hot oil bath to cause reflow of the solder which has been preapplied to the elements and the substrate or just the substrate. As the solder is heated in the bath to above its melting point, it reflows forming a single solder connection between the element and the substrate. The elements or substrates are removed and cooled to provide attachment as well as electrical interconnection.
In conventional reflow methods, the parts are positioned on the substrate at a specific location and held in place by a clamping device or fixture. The pre-assembled substrate element is then immersed in the bath for the reflow process. To assure adequate soldering of the elements to the substrate, solder flux paste is placed on the areas where the solder will reflow. The use of the flux leaves a residue which is often hard to remove in a cleaning process and thus is undesirable. Also, the clamping device increases the mass to be heated, thereby extending the heating time or requiring higher temperatures to reduce the heating time, and causes uneven heating of the parts.
The clamping fixture is specifically designed for a given layout and has to be modified for each specific layout of the elements on the board. This requires an increased inventory of clamping fixture for each substrate layout. Because the clamping fixtures securely hold the elements in place once optically aligned, the elements are not allowed to reposition themselves on the circuit board due to the surface tension of the molten solder. Thus by capillar action, the molten solder will move relative to the fixed element and substrate. This produces undesirable solder patterns and configurations.
Reworking of substrates having elements secured thereto is also a problem faced by the prior art. Generally, the substrate with the elements thereon are subjected to a high temperature gas which causes solder reflow. The individual elements are then removed and replaced in a rework sequence. The high temperature gas causes undesirable thermal shock of the substrate and the elements. For hermetically sealed packages, this often results in hermetic failure and reliability problems. Other undesirable effects of the high temperature gas reflow process are: uneven heating, oxidation of the solder, use of excessive heat to reduce heating time because of poor thermal conductivity, lack of surface tension control of the solder during resoldering, and increased cleaning requirements and charring resulting from the use of flux.